This invention relates to ionization gauges and, in particular, to a method and apparatus for improving the safety and extending the range of such gauges.
The Bayard-Alpert (BA) types of ionization gauge is more than likely the most popular of the high vacuum measurement transducers. Recent tests conducted by the present applicant have, however, disclosed a danger to personnel associated with certain conditions of use of this gauge. In particular, the present applicant has discovered the danger is caused by a conductive plasma which flows from the gauge into the vacuum chamber associated with the gauge. The plasma electrocally couples the metal parts of the chamber to the most positive electrode of the gauge, typically the grid thereof. That is, at pressures of 0.1 Pa, and greater, a BA gauge typically generates sufficient plasma that significant coupling can occur between the grid of the BA tube and the metal parts of the vacuum chamber. This is especially severe during electron bombardment degassing, which involves both glow discharge and vacuum arc mechanisms. This coupling causes significant currents to flow through the ground between the vacuum chamber and controller chassis. Loss of the vacuum chamber ground connection at these pressures floats the chamber at a voltage near that of the BA gauge grid. Contact between ground and the vacuum chamber could be lethal under these conditions of open ground and high pressure.
To investigate the foregoing phenomena, the present applicant arranged a vacuum system as shown in FIG. 1, which includes a mechanical fore pump 10, a diffusion pump 12, a valve V2 connected between pumps 10 and 12, a cold trap 15, valves V1 and V3 where test gas from a source not shown was applied through V3, and ionization gauge G1 and gauge G2, G1 being the gauge in which the phenomena was investigiated and G2 monitoring the pressure in G1.
FIG. 2 illustrates a prior art ionization gauge 16, which may correspond to gauge G1 of FIG. 1 where the gauge may be connected to a vacuum system such a system 14 as indicated in FIG. 6. Gauge 16 includes a filament 18 and a grid 20, the latter electrodes being of primary interest. The collector electrode and its associated electrometer circuit are not shown in FIG. 2 because they do not enter significantly into the gas dishcarge interactions under investigation. Also shown in FIG. 2 is a block diagram of a conventional gauge controller indicated at 22 including a filament power circuit 24, a filament bias circuit 26 which typically provides a filament bias of 30 volts, and a degas power circuit 28, and a ground 30 for the controller.
The normal operation of an ionization gauge requires the generation of ions and liberated electrons of quantities proportional to the pressure being measured. This ionization is not usually thought of as gas discharge formation because there is no visible glow such as is seen in neon signs, florescent lights, glow tubes, etc., which are familiar discharge models. It seems normal one might start to see some discharge glow when the pressure nears 10.sup.-3 torr, for this is the pressure range of the glow discharge. The brightness and color of the glow will depend upon the pressure and the composition of the gas, for each gaseous element has a different spectrum of light given off as its excited ions and molecules return toward normal states after being hit by accelerated electrons.
As the pressure is raised in the measurement mode of operation, ionization gauge controller 22 should turn off tube filament 18 before the glow becomes very bright. Although the plasma in this situation is normal, as it becomes more dense from the increasing pressure, it interferes with the gauge function, making the gauge reading a less linear indication of the pressure. At a sufficiently high pressure, the gauge reading decreases, and eventually changes sign and indicates less than zero pressure. The filament 18 is automatically turned off by most controllers before this can happen. In some controllers the electron current from filament 18 to grid (anode) 20 is measured as a second safety check. If this current deviates from a programmed value, the filament is turned off. If this comparison circuitry is not used, the filament can remain on if it is accidentally turned on at any pressure sufficiently beyond the ion current reversal pressure. This can sometimes result in the burn-out of the filament, especially in those gauges with tungsten filaments.
Of major concern is the safety of the operator and those who touch the controller 22 and vacuum system 14. The generation of high plasma densities in the measurement mode of operation is not typical due to the automatic turn-off of the tube. Even the failure of the ground circuit somewhere between the controller and the vacuum system is not usually of extreme danger in this mode, unless the pressure is a millitorr, or greater. However, in this pressure range, voltages of up to 160 volts can give a brutal shock to an unwary operator who reestablishes this ground circuit with his body. This voltage is sufficient to cause filbrillation of the human heart, so represents a significant danger. Maintaining proper ground circuits on both the chamber and the controller avoids this danger.
Operation of the test equipment of FIG. 1 will now be described with respect to the degas mode of operation of gauge G1. Degas circuits available for cleaning ionization gauges are of two types, namely, electron bombardment (EB) and resistance (I.sup.2 R). EB degas operation is similar to the measurement operation, except that the grid voltage is raised from 180 volts to as high as 900 volts. Also, the emission current is increased from 10 mA to as much as 180 mA. There is now enough power to do significant cleaning. However, there is also enough power to be dangerous if something goes wrong. The EB degas grid voltage is typically the output from a high voltage transformer (not shown) that is full wave rectified, but not filtered. It is dc, but falls to nearly zero 120 times per second. The peak voltage can be as high as 900 V in some controllers.
The 30 V filament bias prevents electrons from reaching ground where the tube is in normal measurement operation. This bias keeps the electrons from interfering with the measurement of ion currents at the collector electrode which is operated near ground potential. In some controllers this filament bias circuit is not operated while degassing.
The high voltage and high emission current in the EB degas mode create much more plasma than is present in the measurement mode of operation. Thus, it is fully expected some purple glow will occur during the early stages of degassing a dirty gauge. The degas operation may be started in the 10.sup.-5 torr range, or lower, but the local pressure in the tube can increase significantly as the contaminating materials evaporate from the tube elements and nearby surfaces. Questions arise as to how much can the pressure be increased and what happens if the pressure is increased too much.
Using the equipment of FIG. 1, gauge G1 was degassed at low pressures. Valve V1 was then clsoed and valve V3 slightly opened with the EB degas power still applied. The pressure increased steadily, and eventually the emission or degas fuse (not shown) would blow. This implied that excessive emission currents had occurred.
Using an oscilloscope, the grid voltage was observed at a point A in FIG. 2, with respect to ground. Simultaneously, the current flowing through point A was monitored. The grid current during the moment before fuse failure was much higher than that which flows normally. This overcurrent was used as a trigger to take pictures of the fuse destructive half cycle.
FIG. 3 shows a few cycles of conventional operation in the 10.sup.-6 torr range at about 40 watts of degas power. The current was about 70 mA except for the short time periods when the grid voltage was too low to support this current. The grid voltage peaked at 780 V in this controller. The pressure was then increased gradually into the 10.sup.-3 torr range where, with degas power applied, the destructive half cycle associated with blowing the emission fuse would occur.
FIG. 4 illustrates the destructive half cycle. To obtain this picture, the oscilloscope scan rate was one msec/division. The current sensitivity was now 5 amperes per division, versus 50 mA used in FIG. 3. The current was as high as 140 times that in normal operation. The grid voltage was very nearly constant at about 320 volts over much of the cycle. This nearly constant voltage was observed as currents varied from 200 mA (the scope trigger) up to about 4 amperes. This behavior was like that of older glow discharge voltage regulator tubes (OA3, etc.), but the current was many times higher than was permissible in those tubes. This mode was called the voltage regulator (VR) mode in the ionization gauge tube for purposes of these tests.
Following the voltage through the destructive half cycle, it suddenly dropped to almost zero, and the current increased more rapidly. This was a vacuum arc. This mode was called the arc mode. The arc and voltage regulator modes are violent, and not very stable, as indicated in FIG. 4. Thus the discharge changed back and forth between them. In dozens of pictures, no two patterns were totally identical.
As can be seen from FIG. 4, the arc sometimes had higher voltage versus ground than at other times. Consequently th current of A and B of FIG. 2 was measured. All of the current went through point A, but only about 10% of the voltage regulator current went through point B. The higher voltage arcs had full current through B, the lower voltage arcs passed no current through B.
Measurement of current through point C in the ground lead proved the point. Both the VR discharge and the low voltage vacuum arcs were to ground--not to other gauge electrodes! This requires plasma contact through many inches of glass tube. The currents involved in these ground return discharges were many amperes--in spite of 1/4 ampere fast blow fuses.
The fuse blowing was always near the end of the half cycle. The fuse appeared to melt sooner than this, but then seemed to arc across the melt until the current fell to a low value near the end of the half cycle. This still is not dangerous if good grounds are present. However, the following question arises. If these currents pass through the common ground of grounds 30 and 32, what happens if this connection is missing, or unable to handle these large currents?
To test this, the ground connection 32 (FIG. 2) was broken to the vacuum system 14 but the controller was left in the rack with a proper ground. A voltmeter (not shown) was placed between vacuum chamber 14 and ground. The controller operated the ion gauge in the measurement mode, and in the degas mode to full pwoer (80 watts) with no more than about 20 volts of ac or dc appearing between the underground vacuum chamber and ground. Then the pressure was increased.
In the degas mode, at pressures of approximately 1.times.10.sup.-3 torr for all gases tested, the voltmeter jumped to 740 volts (peak). The vacuum chamber was +740 volts relative to ground, and the fuse did not blow nor the circuit turn off. After the pressure was increased to the mid 10.sup.-3 torr range controller 22 would automatically turn off due to its emission current comparison circuit. If the pressure stabilized slightly below this, however, the chamber would remain at +740 volts until the degas circuit was manually turned off. Contact between ground and the vacuum system during this time could very possibly have been fatal.
Gauge G1 was a typical (BA) ion gage with a typical controller 22--both in good oeprating condition. The only fault was the missing ground wire to the vacuum system. The gauge tube pressure was no higher than might occur in a typical system when degassing the tube. But this could have killed anyone who made contact between the vacuum system and ground. This is graphically illustrated in FIG. 5 where plasma coupling between grid 20 and vacuum system is indicated at 34 and the current flow indicated by the arrows.
This test was repeated with the same controller with its chassis isolated--that is, disconnected from ground 30 and the vacuum system grounded. In this configuration the controller was driven to -740 volts by this overpressure reaction. Contact between the controller and ground could then have been fatal, also. All tubes and controllers tested gave very similar results.
The resistance type of degas circuit drives current through the wire of the grid, or through an auxiliary heating element. This is high current, usually about 10 amperes, but it uses very low voltage. The heated element typically glows orange. The gauge filament is not functional during this I.sup.2 R degas operation in some controllers, but the grid voltage remains on in most of them. The present applicant has not observed a dangerous interaction between plasma and BA tubes while degassing using this type of controller. However, when the filament and grid are both operational during I.sup.2 R degassing, a plasma coupling problem can again occur. This involves the same situation of current flow to ground as discussed before, even though the full VR mode voltage has not been applied. The filament emission seems to play a critical role in establishing and maintaining this auxiliary discharge. The voltage involved is typically 150 V at a few milliamperes. This will not blow the fuses, nor readily provide other evidence of its presence. Such a discharge may seem mild in compairson with the EB degas case, but it is still deadly in the absence of correct grounding, for it can fibrillate the human heart.
All of these ion gauge controller types are safe when correctly grounded. It is only the failure to have an available current path between the grounded chassis of controller 22 and the exposed metal parts of vacuum system 14 that provides danger. The danger is that the body could then provide such a pathway at any time while the gauge is encountering conditions that could couple power into that pathway, as illustrated in FIG. 5. The result could cost an operator his life.
All manufacturers insist on good grounding for their equipment. Consequently, those in the laboratory and production areas tend to assume this equipment will very seldom fail either (1) in a way that involves the safety grounds that are provided, or (2) in a manner that makes it obvious the safety ground is involved. It is perhaps even more unexpected to recognize a natural phenomenon (the plasma path 34) that leads to large currents through the safety ground circuits 30 and 32--especially one that is easily achieved in normal practice, and can involve voltages and currents of lethal significance.
A safety ground on most electronics equipment is not typically carrying intentional current flow. Thus its potential may differ by several volts from the ground of those vacuum systems which use the power common line as their ground. These two ground systems should have a common junction which is typically at the power distribution breaker box. See FIG. 6. Even though the resistance between these two grounds may be very low, and thus safe, any voltage difference resulting from unbalanced current flow in the vacuum system common lead wil complicate the use of the conventional ohm meter for verifying that low resistance. Even if there is no voltage difference, a resistance measurement accomplished with a few mA may give no clue to some of the problems detectable with more sginificant test currents. Accordingly, existing vacuum chamber/ionization gauge controller chassis grounding systems should be checked to establish they are complete and capable of supporting at least 10 amperes. The placement of a second ground wire (dashed line in FIG. 6) between the vacuum chamber and the gauge controller chassis is not a safe answer, for large continuous currents could flow through it.